The present invention relates to an erasable programmable read-only memory (hereinafter simply called EPROM) device of the type which can be erased by an ultra-violet (UV) ray.
An example of such an EPROM device including a ceramic dual-in-line package (hereinafter simply called ceramic package) has been well known as illustrated in FIG. 1. FIG. 1 is a perspective view of a prior art EPROM device. FIG. 2 is a cross-sectional view along section line A--A of FIG. 1. FIG. 3 is a partially enlarged view of FIG. 2.
In FIGS. 1 to 3, the EPROM device includes a ceramic header 1 formed of a base material 2, leads 3, a low melting temperature glass 4 and a ceramic cap 5 formed of the base material 2. The base material 2 is alumina (Al.sub.2 O.sub.3).
A memory element 7 and a ground die 9 including a head portion 8 covered with evaporated aluminum are bonded or mounted on a chip mounting portion 6 of the header 1. After wire-bonding with thin metal wires 10 such as aluminum between the memory element 7 and the head portions 8, the header 1 with the cap 5 thereon is carried into a sealing furnace set at the melting point temperature of the low melting temperature glass 4, for example, 400.degree. C. to 500.degree. C., so as to seal the ceramic package by melting the low melting temperature glass 4, thereby forming the EPROM device. A window 11 through which an UV ray passes to the memory element is formed in the center of the cap 5.
According to the above mentioned prior art device, the aluminum thin metal wire 10 which electrically connects a ground electrode pad 12 of the memory element 7 to the substrate of the memory element 7 through the chip mounting portion 6 cannot be directly connected to the chip mounting portion 6 which is sintered with gold paste, because the sealing temperature of the glass is relatively high. Accordingly, aluminum thin metal wire 10 must be relayed by the head portion 8 of the ground die 9 which is a piece of silicon, and this is a difficult operation. Though a ceramic package is one of the most low-priced packages available, it has other defects due to the high sealing temperature, other than that mentioned above. One defect is that construction requires substantial time because gold wires cannot be used for wire-bonding. Another defect is that cracks can be formed in the package because the ceramic package is quite fragile with regard to shocks. Furthermore, the ceramic package is relatively heavy, thereby restricting the package density on a printed circuit board.
Examples of plastic mold type EPROM device for overcoming such defects have been proposed in Japanese application Nos. 183085/1983 and 183086/1983.
However, then still have some defects, namely:
(1) Since an UV ray permeable resin has relatively low adhesive strength and water tightness, the moisture resistance of the package is not always satisfactory. Additionally, since the plastic package has relatively low resistance to thermal stress, the quality of the package sometimes is degraded by a heat cycle test.
(2) Since the resin used in the plastic package is rather expensive, even though it provides sufficient transmittal of the UV ray, it is desired to reduce the quantity of such resin to be used.
(3) The difference in thermal expansion coefficient between sealing resin by which all packages are enclosed or the base material (alumina) and the UV ray permeable resin sometimes generates cracks in the UV ray permeable resin.